A major goal of ecology and evolutionary biology is to explain geographic patterns of species richness. Richness is often correlated with climatic variables. However, the processes underlying these climate-diversity relationships remain poorly understood. Two potential hypotheses to explain these relationships involve: (i) faster diversification rates (speciation minus extinction) in high-richness climates, and (ii) earlier colonization of high-richness climates, allowing more time for speciation to build up richness. Few studies have tested these hypotheses directly, and most focused on animal clades with limited richness. In this study we test these hypotheses in Chinese angiosperms, encompassing ~10% of Earth’s plant species, using large-scale phylogenetic, climatic, and distributional data including 26,977 species. We find that climatic zones that were colonized earlier have higher species richness. In contrast, relationships between diversification rates and richness of climatic zones are often non-significant or negative. Our study reveals that even when richness is strongly correlated with climate, the underlying explanation may still be rooted in phylogenetic history. We also show that the timing of colonization can be crucial for explaining richness patterns. Yet, most recent studies have ignored this explanation and instead have focused solely on rates of speciation and diversification as drivers of diversity gradients.